Pokemon Showdown's Damage Calculator uses an incorrect modifier for Tough Claws; it uses 30% boost instead of 33% boost. Here is what it lists for my example:
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 153-181 (39.8 - 47.1%) -- guaranteed 3HKO
(153, 156, 157, 159, 161, 162, 165, 166, 168, 170, 171, 174, 175, 177, 179, 181)
252 Atk Life Orb Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 153-181 (39.8 - 47.1%) -- guaranteed 3HKO
(153, 155, 156, 159, 160, 162, 164, 166, 168, 169, 172, 173, 175, 177, 179, 181)
Notice how Tough Claws boost is similar to Life Orb boost. Here is what it should be:
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 156 - 185 (40.6% - 48.1%) -- 67.2% chance to 2HKO after Stealth Rock
(156, 159, 160, 162, 164, 166, 168, 170, 171, 174, 175, 177, 179, 181, 183, 185)
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 153-181 (39.8 - 47.1%) -- guaranteed 3HKO
(153, 156, 157, 159, 161, 162, 165, 166, 168, 170, 171, 174, 175, 177, 179, 181)
252 Atk Life Orb Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 153-181 (39.8 - 47.1%) -- guaranteed 3HKO
(153, 155, 156, 159, 160, 162, 164, 166, 168, 169, 172, 173, 175, 177, 179, 181)
Notice how Tough Claws boost is similar to Life Orb boost. Here is what it should be:
252 Atk Tough Claws Mega Charizard X Flare Blitz vs. 172 HP / 0 Def Azumarill: 156 - 185 (40.6% - 48.1%) -- 67.2% chance to 2HKO after Stealth Rock
(156, 159, 160, 162, 164, 166, 168, 170, 171, 174, 175, 177, 179, 181, 183, 185)